A visible light image photographed at a time of foggy or hazy weather becomes whiter or has low contrast due to particles existing in the air, and consequently quality of the visible light image is degraded. As illustrated in FIG. 15, under a foggy or hazy environment, reflected light which is reflected from an object of photographing is diffused and attenuated by particles which exist in the air on a path from the object of photographing to a camera. Moreover, at the same time, environmental light is also diffused by the particles existing in the air, and the diffused light reaches an image sensor of the camera. Therefore, light, which the image sensor of the camera receives, is mixed light of the attenuated reflected-light which comes from the object of photographing, and the diffused environmental light. In order to eliminate effects due to the particles existing in the air from the visible light image in which the object of photographing is photographed, it is necessary to eliminate effects of the diffused environmental light from the mixed light, and to restore the reflected light that has been attenuated due to the particles existing in the air.
Until now, a method using information of a near infrared image, in which a transmittance in the foggy or hazy air has a large value and which has a small amount of degradation, is proposed in order to realize restoration to a visible image having no degradation.
Patent literature (PTL) 1 discloses an art that the visible light image is divided into a luminance signal and a color difference signal, and the visible light image is emphasized by combining a near infrared signal with the luminance signal. PTL 2 discloses an art that, by replacing a luminance signal of an object image with a long-wavelength luminance signal in a low chroma area where a difference between a contour part of the long-wavelength luminance signal and a contour part of a visible light luminance signal is equal to or larger than a predetermined value, the visible light image is emphasized.
Each of the arts which are described by PTL 1 and PTL 2 is related to processing of emphasizing the luminance signal, and consequently visibility of the emphasized image becomes improved. However, improvement of image quality regarding color information or the like is not planned.
Meanwhile, the non-patent literature (NPL) 1 is known as image processing for improving the image quality including the color information. NPL 1 uses prior knowledge that, at a time of sunny weather, there is a pixel, in which one of color channels has a null value, around each pixel. At a time of restoration from degradation, performance in restoring the visible light image is improved by imposing restriction that a local gradient of a restored visible light image and a local gradient of the near infrared image are similar each other.
Here, PTL 3 describes an art of spectrophotometric measurement of measuring a fluorescence wavelength band, and a long wavelength band except for the fluorescence wavelength band in a specimen including a fluorescent substance. Moreover, PTL 4 describes an art for coinciding a color of a printed matter, which is observed by reflected light, with a color of a light source which is displayed on a screen. Furthermore, PTL 5 describes an art of converting an infrared image into a visible image or a near infrared image.